I had a huge effing problem with online check in (4 calls to the Diamond line with mixed results) and then subsequently issues with my GUC. The red coat I talked to said fare class is the top factor in determining priority. She didn't seem to care that I had applied the GUC back in May. No upgrade for me.
Ironically, OLCI did not work for me at all yeaterday. Took me nearly two hours to actually get checked in at the airport, but that's another story entirely.
Based on everything I learned yesterday about GUC priority, I can say somewhat difinitively at this point that following ordering applies:
1) Status
2) Fare Class
3) time of request
Other things may come into play, but those are the ones, in that order, that I actually have some data to back up.
It also appears that it does not make a difference in certificate type (I.E., GUCs and RUCs appear to have the same priority. Had pretty good evidence that a DM with a RUC but a higher fare class was ahead of me on a domestic leg where I had a GUC listed).
The new companion rules DO apply to GUCs and RUCs when two people list together (must be same PNR) using certs. I.E. a DM and a no status companion both on RUCs would both trump a PM with a cert.